The invention generally relates to a fuel processor design and a method for manufacture.
A fuel processor is a device that converts a hydrocarbon fuel into hydrogen. Examples of hydrocarbon fuels include natural gas, gasoline, methanol, etc. A common fuel processor application is to supply hydrogen to a fuel cell system, where the hydrogen is reacted to produce electricity. For example, fuel processors typically provide an output stream, referred to as reformate, that consists primary of hydrogen, carbon dioxide, water and nitrogen (from the air used to react the hydrocarbon). Exemplary fuel processor systems are described in U.S. Pat. Nos. 6,207,122, 6,190,623, 6,132,689, which are hereby incorporated by reference.
The two reactions which are generally used to covert a hydrocarbon into a reformate stream are shown in equations (1) and (2).½O2+CH4→2H2+CO  (1)H2O+CH4→3H2+CO  (2)
The reaction shown in equation (1) is sometimes referred to as catalytic partial oxidation (CPO). The reaction shown in equation (2) is generally referred to as steam reforming. Both reactions may be conducted at a temperature from about 600-1,100° C. in the presence of a catalyst such as nickel with amounts of a noble metal, such as cobalt, platinum, palladium, rhodium, ruthenium, iridium, and a support such as magnesia, magnesium aluminate, alumina, silica, zirconia, by themselves or in combination. Alternatively, reforming catalysts can also be a single metal, such as nickel or platinum, supported on a refractory carrier like magnesia, magnesium aluminate, alumina, silica, or zirconia, by themselves or in combination, or promoted by an alkali metal like potassium. As an example, a platinum wash-coated ceramic monolith may be used. As further examples, catalyst pellets may be used, which may be held in a flow-through reactor canister by screens. Catalyzed plate heat exchangers may also be used. Catalyzed shell and tube heat exchangers may also be used, for example, with tubes catalyzed either internally or externally.
A fuel processor may use either of these reactions separately, or both in combination. While the CPO reaction is exothermic, the steam reforming reaction is endothermic. Reactors utilizing both reactions to maintain a relative heat balance are sometimes referred to as autothermal (ATR) reactors (note that the terms CPO and ATR are sometimes used interchangeably). Also, it should be noted that fuel processors are sometimes generically referred to as reformers, and the fuel processor output gas is sometimes generically referred to as reformate, without respect to which reaction is employed.
As evident from equations (1) and (2), both reactions produce carbon monoxide (CO). Such CO is generally present in amounts greater than 10,000 ppm. Because of the high temperature at which the fuel processor is operated, this CO generally does not affect the catalysts in the fuel processor. However, if this reformate is passed to a fuel cell system operating at a lower temperature (e.g., less than 100° C.), the CO may poison the catalysts in the fuel cell by binding to catalyst sites, inhibiting the hydrogen in the cell from reacting. In such systems it is typically necessary to reduce CO levels to less than 100 ppm. For this reason the fuel processor may employ additional reactions and processes to reduce the CO that is produced. For example, two additional reactions that may be used to accomplish this objective are shown in equations (3) and (4). The reaction shown in equation (3) is generally referred to as the shift reaction, and the reaction shown in equation (4) is generally referred to as preferential oxidation (PROX).CO+H2O→H2+CO2  (3)CO+½O2→CO2  (4)
Various catalysts and operating conditions are known for accomplishing the shift reaction. For example, the reaction may be conducted at a temperature from about 300-600° C. in the presence of various catalysts including ferric oxide, chromic and chromium oxides, iron silicide, supported platinum, supported palladium, and other supported platinum group metals, by themselves or in combination. Other catalysts and operating conditions are also known. Such systems operating in this temperature range are typically referred to as high temperature shift (HTS) systems.
The shift reaction may also be conducted at lower temperatures such as 100-300° C. in the presence of other catalysts such as copper supported on transition metal oxides like zirconia, zinc supported on transition metal oxides or refractory supports like silica or alumina, supported platinum, supported rhenium, supported palladium, supported rhodium and supported gold, by themselves or in combination. Combinations of copper with cerum or rare earth metals or ceria or rare earth metal oxides are also know to exhibit high catalytic activity. Such systems operating in this temperature range are typically referred to as low temperature shift (LTS) systems. LTS reactors often utilize catalyst pellets. Other catalysts and operating conditions are also known. In a practical sense, typically the shift reaction may be used to lower CO levels to about 3,000-10,000 ppm, although as an equilibrium reaction it may be theoretically possible to drive CO levels even lower. In the context of the present invention the term “shift reactor” is sometimes used generically to cover any shift reactor configuration including HTS, LTS and others.
The PROX reaction may also be used. The PROX reaction is generally conducted at lower temperatures than the shift reaction, such as 100-200 □C. Like the CPO reaction, the PROX reaction can also be conducted in the presence of an oxidation catalyst such as platinum. The PROX reaction can typically achieve CO levels less than 100 ppm. Other non-catalytic CO reduction and reformate purification methods are also known, such as membrane filtration and pressure swing adsorption systems.
In some systems, it may be desirable to further include a desulfurization stage placed upstream from the fuel processor to remove sulfur compounds from the fuel before it is reacted (e.g., to avoid poisoning the catalysts of the fuel processor and/or the fuel cell stack). For example, activated carbon, zeolite, and activated nickel materials are all known in the art for such application.
As previously indicated, hydrogen fuel cells are a common fuel processor application. A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:H2→2H++2e− at the anode of the cell, andO2+4H++4e−→2H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage of up to about one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow field plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair of electrodes are often assembled together in an arrangement called a membrane electrode assembly (MEA) or a membrane electrode unit (MEU) when the GDLs are included.
For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. The amount of a reactant supplied may be referred to in terms of “stoich”. For example, for a given electrical load on a fuel cell, one stoich of hydrogen and one stoich of air would refer to the minimum amount of each reactant theoretically required to produce enough electrons to satisfy the load (assuming all of the reactants will react). However, in some cases, not all of the hydrogen or air supplied will actually react, so that it may be necessary to provide excess fuel and air stoichiometry so that the amount actually reacted will be appropriate to satisfy a given power demand.
Hydrogen that is not reacted in the fuel cell may be vented to the atmosphere with the fuel cell exhaust, and in some cases may be oxidized before it is vented. Such exhaust may also contain small amounts of hydrocarbons that “slip” through the fuel processor without being reacted. Substantial heat may be generated as these exhaust components are oxidized, for example by mixing them with air and passing them through a platinum-coated ceramic monolith similar to an automotive catalytic converter.
There is a continuing need for fuel processor designs addressing concerns and objectives including the foregoing in a robust and cost effective manner.